Thermoplastic polymers can be used for diverse applications, for example in the packaging industry. Here, the polymers are often mixed with fillers to improve very diverse material properties. For example, titanium oxide pigments are employed in particular in PET beverage bottles to achieve imperviousness to visible light, thus increasing the durability of light-sensitive food, such as milk. Furthermore, coloring pigments are often used to modify the optical appearance of the bottles. Additives are also often added to influence gas diffusion. The fillers are dispersed in the polymer matrix as finely and regularly as possible to achieve an optically homogenous mixture and not to deteriorate the mechanical material properties. This is conventionally done by master batch technologies and/or the use of surface-modified fillers.
The good dispersion of the fillers in the thermoplastic polymers, however, leads to problems in conventional recycling processes of these materials as the fillers can be only insufficiently removed from the recyclate.
For recycling, the materials are usually collected after they have been used, sorted according to the material types by mechanical and physical separation methods, then cut into smaller pieces, the so-called polymer flakes, and washed. These polymer flakes represent an intermediate in the recycling process and are transformed again to polymer granules by a continuous extrusion procedure which can be reshaped again to any arbitrary products.
The remaining coloring pigments in the recyclate deteriorate its quality and recyclability as only products can be manufactured from it which have a darker hue than the recyclate. On the other hand, it is required for many applications to obtain a colorless, transparent product. To suppress this negative effect, a mechanical filter is usually provided in the extrusion and granulation device by means of which fillers are to be separated from the polymer melt. However, the particle sizes of finely dispersed fillers can already be within the range of the polymer macromolecules, so that they can not or only insufficiently be removed from the polymer melt by the mechanical filtering step.
DE 4112786 A1 and JP 2007/246833A describe methods of separating fillers from polymer solutions, however without disclosing the separation from a polymer melt. US 2010/0249253 A1 relates to a method for recycling polyvinyl butyral in laminated glass, comprising chemical purifying steps, where the polyvinyl butyral is present in the solid phase. These methods are relatively complicated and ineffective as they require the preparation of a polymer solution or chemical treatment.